Layer 2 services such as Frame Relay, Asynchronous Transfer Mode (ATM), and Ethernet can be emulated over a Packet-Switched Network (PSN) by encapsulating the Layer 2 Protocol Data Units (PDUs) and transmitting them over pseudo-wires (PWs). Pseudo-wire Emulation Edge-to-Edge (PWE3) defines a mechanism that emulates the essential attributes of a Layer 2 service (e.g., ATM, Frame Relay, Ethernet, etc.) over a PSN. With PWE3, a connection between Customer Edge (CE) devices located at geographically dispersed sites can be emulated using a PW. The PW emulates the transactions of a wire connecting the CEs.
A Provider Edge (PE) device provides one or more PWs on behalf of the CE devices it serves so that the CE devices can communicate over a PSN. A PSN tunnel is established to provide a data path for the PW. Native service PDUs are encapsulated and carried across the PSN via the PSN tunnel. The PE devices perform the necessary encapsulation and de-capsulation to make communication over the PW transparent to the CEs. The encapsulation includes a PW label that is used to identify the PW in the data plane. The encapsulation further includes a transport label that is used for forwarding traffic in the data plane. The PE devices are responsible for setting up, maintaining, and tearing down PWs. This includes the exchange of PW labels and transport labels between PE devices.
Conventionally, at least three different protocols are required to support PW functionality in Layer 2 Virtual Private Network (L2VPN) deployments. The first protocol is an Interior Gateway Protocol (IGP) for distributing network reachability information within the PSN (e.g., Intermediate System to Intermediate System (IS-IS) or Open Shortest Path First (OSPF)). The second protocol is a protocol for distributing transport labels (e.g., Label Distribution Protocol (LDP)). The third protocol is a protocol for exchanging PW labels (e.g., Targeted Label Distribution Protocol (Targeted LDP) or Border Gateway Protocol (BGP)). With the introduction of segment routing, an IGP can be used to exchange the transport labels. However, L2VPN deployments still require two separate protocols to support PW functionality (e.g., IGP and Targeted LDP/BGP). Employing multiple protocols to implement PWs can be operationally complex to configure, maintain, and troubleshoot.